Pads, density, effect

Good separation efficiency is obtained from 0.3V, to Va. Efficient removal of smaller particles increases with velocity until re-entrainment occurs at velocities above V . Mesh pads are effective in drop removal down to 5-10, [im. Some solids may be handled, hot the lower-density mesh and liquid sprays above the ped ate recommended to prevent pluggage. 

Luo and Domfeld [110] introduced a fitting parameter H , a d5mamical" hardness value of the wafer surface to show the chemical effect and mechanical effect on the interface in their model. It reflects the influences of chemicals on the mechanical material removal. It is found that the nonlinear down pressure dependence of material removal rate is related to a probability density function of the abrasive size and the elastic deformation of the pad. 

Hence, Tct is seen to increase with pore density and pore radius. However, a problem appears at a porous substrate when thin films are to be deposited during metallization to form interconnections, thin-film capacitors, etc.335 Sputtered material falls deep into the pores, which affects the planarity of the deposited layer and the electrical resistivity of the oxide layer underneath.335 To cope with this effect, the porous oxide should be padded by inorganic (A1203 and Si02) or organic (polyimide, negative photoresist) layers. 

In the simplest case, a square area can be used to determine the effective density across the mask, as shown in Fig. 11. Density to be assigned to the coordinates at the center of the window is equal to the ratio of raised to total area of the square window. The length of each side of the square is then defined as the planarization length this square region approximates the deformation characteristics of the pad and process. The size of the square (or the planarization length) is determined experimentally by varying the square size until the effective density calculation results in predicted thickness values that best fit experimentally measured polish data when used in the thickness evolution model. 

The actual limit of the summation is the extent of the weighting filter. Zero padding is used to ensure that the discretized matrices have sizes which are a power of two so that the computation can be done in the frequency domain using fast fourier transform (FFT) techniques. The effective discretized density, pin, Wj), is then given by... 

Figure 17 shows the effective density using the elliptic filter with a characteristic length of 2.9 mm. The optimal length must be determined for each consumable set and process conditions since the planarization length is dependent not only on the polish pad type but also on the polish process conditions, notably the down force. 

Removal or elimination of mycotoxins. Since most of the mycotoxin burden in contaminated commodities is localized to a relatively small number or seeds or kernels [reviewed in Dickens, 200], removal of these contaminated seeds/kemels is effective in detoxifying the commodity. Methods currently used include (a) physical separation by identification and removal of damaged seeds, mechanical or electronic sorting, flotation and density separation of damaged or contaminated seeds (b) removal by filtration and adsorption onto filter pads, clays, activated charcoal (c) removal of the toxin by milling processes and (d) removal of the mycotoxin by solvent extraction. 

The expression in Equation 5.4, and the model it is based on, uses two values to parameterize the interaction between pad and abrasive UoPad and Apad- It is reasonable to ask about [76] the effect of abrasive size on each of these. If there is little separation between sites, as appears to be the case shown in Fig. 5.15, then the site density oPad must decrease with increasing abrasive area, which is proportional to the square of the abrasive diameter d - If, however, the mechanical removal rate per particle increases with the particle diameter, then these effects would largely cancel each other. To understand the effect of abrasive diameter on Apad = koff/kon, it is helpful to think about how particle size affects the rates of attachment to and separation from the pad. The... 

Figure 5 illustrates endpoint time versus pad wear. In order to eliminate the effect of topological density differences, pad life wear versus endpoint time was examined on two different device types. Note Typical pad changes occur after a wafer count of 400. 

The ability to accurately model die pattern evolution as discussed in this paper provides a solution applicable to the ran by run control of multi-product patterned wafers [13]. As shown in Fig. 10, a feedback control loop incorporating the integrated density and step-height pattern dependent model was developed. For each device type, an appropriate set of model parameters (including effective blanket rate BR and planarization length) were determined. The model for the effective blanket rate includes a term Delta(n) that is updated on each run n to track the tool drift in rate over time due to pad and consumable wear ... 

Two probability distributions are most likely to be important (a) the cell density probability distribution (which is the distribution of cell center locations) and (b) the cell size distribution (around the cell center locations). The combination of these distribution functions, bounded by the material relative density (say, 0.3), should result in the cell wall thickness probability distribution. A postulated cell wall thickness probability distribution can be seen in Figure 6.34a. For a given process at its typical temperature, there will be a wall thickness that will respond to the applied force within wall deflections, and one resulting in wall plastic deformations. Increasing the relative density of the pad material will shift this curve toward thicker walls, with lower probabilities of each of the above mechanisms. The effect of these mechanisms cannot be derived from the material density. 

---